The invention relates to an electrically driven fastening appliance having a clutch mechanism for automatic torque control to discontinue further power transmission from a power source to a driving spindle on receiving a predetermined torque.
Hitherto there were provided and used electrically operated screwdrivers of the type in which an electric motor as power source is located in a grip housing to transmit an output thereof to a driving spindle through a gear system such devices are known from the British Pat. Nos. 853,407 and 1,121,782.
In the tightening operation of a screw-threaded connecting means, such as a screw, with use of an electrically driven screwdriver of the type heretofore used, the total amount of torque to be applied to the screw head just before termination of the tightening operation includes the net motor output increased by the gear system and a kinetic inertia accumulated in accordance with revolutions of the rotor and the rotating elements of the gear system. While the net output component to be derived from the motor is usually constant, the amount of the kinetic inertia depends on the velocity of the rotating elements and is complicatedly variable due to different conditions such as shape and size of the screw shank, hardness of the base material and degree of tightening resistance. The amount of the kinetic inertia in terms of torque also varies in proportion to the square of the turning speed of the driving spindle. Hence an assumption of the amount of the kinetic inertia to be generated in each intended driving operation is difficult. Moreover, although there have been provided various torque controlling means responsive to the output of the motor, unanticipated excessive tightening torque is still generated, as a result of which damage to the screw and also the base material occurs.
In consideration of the foregoings, it has been determined that the exact torque controlling might be achieved by eliminating or possibly reducing undesired impact torque to be generated by accumulation of unstable kinetic inertia to derive an anticipative normal motor output torque. For example, when a screw-threaded connecting means such as a screw is tightened by a certain fastening appliance such as an electrically driven screwdriver, undesired kinetic inertia component only is removed from the total amount of the tightening torque to be applied, just before termination of the tightening operation to the screw head including a normal output component to be derived from the motor driving source plus undesired operation inertia to be added by the motor driving, so that only the normal output torque to be derived from the driving power source may be transmitted to the screw head to operate the clutch mechanism with the reversing torque of an amount equivalent to the output torque thereby to perform an automatic control of the torque.
After extensive studies it has been conceived that a substantial amount of the total reversing torque developed just before termination of the tightening operation may be converted into an energy sufficient enough to start another stationary element to turn, so as to substantially absorb the impace force. As the stationary elment an internal gear constituting a part of a planetary gear train may preferably be utilized to provide a novel torque controlling clutch assembly.